1. Field of the Invention
This invention relates to a cryogenic liquid cylinder and, more specifically, to a cryogenic liquid cylinder having a vertical external pressure building circuit.
2. Description of the Prior Art
Industrial applications, such as laser cutting, often require a large volume of certain gases. To transport such gases to a remote location, cryogenic liquid cylinders are used. When a gas is converted into a cryogenic liquid, a great quantity may be stored in a reduced volume and at a lower pressure compared to the volume and pressure required to store the substance as a compressed gas. Cryogenic liquid cylinders are well insulated and, as a result, the cryogenic liquid does not evaporate quickly while being stored. This can be a drawback, as many industrial applications require a high volume of gas at or near ambient temperature. Accordingly, cryogenic cylinders often include a means to enhance the conversion of the cryogenic liquid into a gas.
To convert a cryogenic liquid to a gas, a cryogenic cylinder is typically equipped with both a pressure building circuit and a vaporizer circuit. Both circuits drain liquid from the lower portion of the container and deliver the liquid to a tubular member which is less insulated than the cryogenic liquid chamber. Because the tubular member is less insulated, the cryogenic liquid's temperature in the tubular member is raised above its boiling point and the liquid is converted into a gas. In the pressure building circuit, the gas is returned to the upper portion of the container. This gas increases the pressure in the container. Gas from a vaporizer circuit is channeled directly to an end application. Once the pressure building circuit provides the desired gas pressure, gas may then be channeled from the cylinder through the vaporizer circuit to the end application. In applications such as laser cutting, the pressure in the cylinder must be maintained above 400 p.s.i. during high continuous flow (&gt;2000 cfh).
Cryogenic liquid cylinders typically have an outer shell or container and an inner container. A vacuum is maintained between the inner and outer container to help insulate the inner container where the cryogenic liquid is stored. Additional insulation may be provided through alternate layers of paper and aluminum foil to reduce heat transfer through conduction and radiation. Presently, pressure building circuits are disposed in the plenum between the inner and outer containers. This location protects the pressure building circuit within the outer container. The pressure building circuit consists of a tube connected at one end to a lower opening through the lower portion of the inner container, and at the other end to an upper opening through the upper portion of the inner container. This allows a quantity of cryogenic liquid to enter the pressure building circuit through the lower opening, whereupon the liquid head, i.e., the pressure created by the higher level of liquid in the container, will force the cryogenic liquid upwards through the pressure building circuit. As the cryogenic liquid passes through the circuit, its temperature is raised until it boils thereby becoming a gas. The gas then passes through the upper opening back into the inner container.
While being located within the outer container protects the pressure building circuit, the location of has several disadvantages that prevent such containers from providing high pressure at continuous flow (e.g. 400 p.s.i. at a flow rate &gt;2000 cfh) and a relatively warm (within 20.degree. F. of ambient) gas. For example, because of the limited space between the inner and outer containers, a typical pressure building circuit consists of 1/4-3/4 inch copper tubing wrapped about the internal container. The tube contacts the outer container and is heated by conduction. This heat transfer provides the energy to convert the cryogenic liquid to a gas. However, because only one side of the tube contacts the outer container, and because the tube may also contact the inner container, the rate of heat transfer is lower than can be achieved with an external pressure building circuit. When the rate of heat transfer is low, the conversion of cryogenic liquid into gas is slow. Additionally, when the rate of heat transfer is low, the temperature of the resulting gas may not rise far above the temperature of the cryogenic liquid.
An internal pressure building circuit has another disadvantage caused by the construction of most cryogenic liquid cylinders. Current welding methods used with cryogenic containers prevent the lower opening to the inner container from being adjacent to the bottom of the container. Additionally, the pressure building circuit will extend upwards to a point approximately 8 inches above the bottom of the inner cylinder. Because the level of liquid in the cylinder must be several inches above the lower opening to provide a sufficient liquid head to force the cryogenic liquid through the pressure building circuit, the pressure building circuit will stop working when the level of cryogenic liquid drops close to the level of the top of the pressure building circuit, is approximately 8 inches above the bottom of the inner cylinder. This is a disadvantage because, when the lower opening is several inches above the bottom of the inner container, a significant quantity of cryogenic liquid remains in the tank after the pressure building circuit loses its functionality.
External pressure building circuits have been used in the past, see e.g. Wildhack, U.S. Pat. No. 2,576,985. However, such external pressure building circuits have some structural similarities to internal pressure building circuits, e.g. a long tubular member wrapped around the cylinder, and similar limitations, e.g. a maximum pressure of 50 p.s.i. See Wildhack FIGS. 7-9 and col. 3, lines 71-73. Additionally, prior art external pressure building circuits used long, narrow tubular members. See e.g. Wildhack at col. 4, lines 24-25, noting a coil length of 80 feet with an inner diameter of 5/8 inch. Wildhack also discusses a high pressure charging converter, see Wildhack cols. 7-8, lines 60-42, but notes that despite an operating pressure of 2000 p.s.i., the flow rate is approximately 48 cu. in./min. and such a gas would not be at or near ambient temperature.
Therefore, there is a need for a pressure building circuit that is capable of maintaining a high pressure during high continuous flow.
There is a further need for a pressure building circuit that is protected from damage.
There is a further need for a pressure building circuit that enables use of substantially all of the cryogenic liquid contained in a cryogenic cylinder.
There is a further need for a pressure building circuit that can fit in a confined space.
There is a further need for a pressure building circuit that can be attached to a portable cryogenic liquid cylinder.